This invention relates to apparatus and method for plasma processing, and in particular to an inductively coupled thermal plasma generator.
Plasma processing is well known in the art, and is currently used in industrial and material processing applications. Plasma torches of various types are described by R. M. Young et al. (Plasma Chemistry and Plasma Processing, 5, 1-37 (1985)), incorporated herein by reference. Thermal plasma is usually generated by one of two standard techniques. The most commonly used method involves the generation of a DC (direct current) arc between appropriately designed electrodes that are bathed in a flowing carrier gas medium. The arc heats the flowing gas, which is then expelled at high velocity from the torch nozzle. The resulting flame can reach temperatures as high as 10,000.degree. C.-20,000.degree. C. The device may be used for many materials processing applications, such as plasma spray coating of various substrates with corrosion resistant, hard surfacing, or thermal barrier layers, as well as spheroidization of refractory powder particles.
A second plasma generating method involves inductive RF (radio frequency) coupling. This technique needs no electrodes in direct contact with the flowing gas. Instead, the tube through which the gas is flowing is surrounded by an inductive coil carrying an RF current, and power is coupled into the hot conductive gas, in a manner somewhat similar to the techniques used in inductively coupled arc lamps. The frequencies used run in the range of fractions to tens of megahertz. These devices are used for applications similar to those for which the DC torches are used, and have been found particularly suited to such applications as chemical synthesis of ceramic or metal powders.
Each method has its own unique advantages and drawbacks. In the DC torch, the arc is in direct contact with the gas, heating it more efficiently. However, the DC torch requires high velocity gas flow, produces high radial temperature gradients, and is subject to electrode erosion, particularly at the electron-emitting cathode. Also, penetration of the stream of hot gas by particles, e.g. in particle spheroidization applications, is relatively difficult to control, with the high viscosity of the rapidly flowing hot gas causing particles to be deflected by the stream.
Inductive coupling, on the other hand, allows greater radial uniformity of temperature, lower gas velocity, and easier and more reproducible particle penetration. Further, with no cathode to erode, there is less chance for product contamination, and reactive gases which would destroy the DC cathodes can be safely used to produce plasma in an inductively coupled torch. This opens new possibilities of producing plasmas from reactive, or even corrosive gases and gas mixtures. All of these advantages, however, are achieved at the expense of coupling efficiency. Most of the power is deposited not at the center line of the gas flow but near its outer periphery, the electrically conducting plasma then partially shielding the gas closer to the center. This typically gives a temperature distribution with some degree of a "hole" with the temperature being higher toward the outer circumference than near the center. Both coupling efficiency and uniformity are frequency-dependent; lowering the frequency allows the power to be deposited progressively closer to the center, but at the cost of a progressively higher power threshold required for plasma generation. This effectively prevents the use of higher-enthalpy gases, for example nitrogen or hydrogen, as the major plasma constituent at any but the highest power levels.
Within the past several years, a number of attempts have been made to combine the two techniques to form a "hybrid" system, enhancing some of the desired characteristics and/or minimizing some of the drawbacks. Typically in such applications, a lower power DC torch is used to generate a flame which is then passed through an inductive coil. This improves the temperature uniformity (effectively filling the thermal "hole" normally found in simple RF torches), while at the same time enabling better control of particulate/reactant input by allowing injection between the two stages. A typical hybrid system is described by Toyonobu Yoshida et al. (J. Appl. Phys., 54, pp 640-646 (1983)). Alternatively, two stages of inductive coupling can be used, with similar results, as described by T. Kameyama et al. (ISPC-8, Tokyo, 1987, Paper No. P-159, pp 2065-2070). The Toyonobu Yoshida and T. Kameyama papers are both incorporated herein by reference.
However, under certain conditions, particularly at higher frequencies, a floating voltage may be generated in the plasma fireball and flame of systems involving RF plasma generation. This floating voltage appears to comprise mostly AC voltage with some DC component, and can involve voltages sufficiently high to produce arcing to conductive components of the torch, even across a gap of several feet, extinguishing the plasma flame. The origin of this anomalous voltage is not well understood, but may be related to the existence of a recirculation eddy within the torch. The floating voltage can often be somewhat controlled by operating at a lower frequency, but this requires a higher power input, often as much as an order of magnitude higher than that required at high frequency operation. Operating such systems at sufficiently high frequencies for efficient operation requires complete electrical isolation of the flame from other torch components, lest an electrical short develop between them, causing arc instability or extinction. In simple RF systems, this isolation can be relatively easy to achieve by designing a sufficiently large gap between the tail flame and any conducting structure surrounding the flame. However in hybrid systems, because of the requirement for dual generation, and in certain more complex RF systems, such isolation can involve complex design considerations focused on preventing generation of this voltage and/or providing the requisite degree of electrical isolation. Such approaches can impose limitations on device performance and/or operating frequency, and are not always successful.
It would be advantageous if, rather than directing efforts toward prevention or isolation of the anomalous voltage, a way could be found to utilize this voltage to improve energy efficiency and spatial temperature distribution. The present invention provides such a way.